Increasing costs for petroleum-derived fuels are driving interest in alternative starting materials (feedstocks) for the production of fuels. Additionally, concern over increasing atmospheric carbon dioxide levels has spawned interest in “carbon-neutral” fuels. One possible solution to both of these issues is the utilization of TAG feedstocks for the production of hydrocarbon-based transportation fuels.
Certain TAGs are already utilized as feedstocks for the production of “biodiesel.” In this process, the TAG is transesterified with methanol, ethanol, or another C1-C5 alcohol, to provide a C1-C5 FAE and glycerine. The C1-C5 FAE is separated, purified, and sold as an additive, supplementing petroleum-derived diesel fuel. C1-C5 FAE diesel additives provide certain specific benefits to their use (e.g., lubricity), but suffer serious physical limitations when used as the sole fuel and not as a blendstock (e.g., cold-flow properties).
C1-C5 FAE diesel fuel represents a first-generation bio-derived fuel. The shortcomings of this generation of fuel are directly related to the fuel-possessing oxygen functionality. A second-generation fuel possesses no oxygen functionality, providing a more petroleum-like product with respect to elemental composition, and is oftentimes termed “renewable diesel.”
Recent publications and patents have described the conversion of TAG to hydrocarbon fuels via technology oftentimes referred to as “hydrodeoxygenation.” This technology converts the fatty acid-portion of a TAG to a hydrocarbon having the same number of carbons as the fatty acid-portion or to a hydrocarbon possessing one carbon less than the fatty acid-portion. The glycerine portion of the TAG is most often converted to propane or otherwise lost within the process.
The glycerine portion of the TAG possesses economic value in itself greater than that of propane and, as such, could be an important economic by-product from an overall process that would provide glycerine as a by-product.
Certain patents list strategies for limiting the acidity of the fuel that is produced. This can include recycle of the product with fresh feedstock over the catalyst bed and limiting the total acidity of the product introduced to the catalyst.
A major difference between a fatty acid and a TAG is the nature of the carboxyl functionality present in each compound. For the TAG, the acid is present as an ester (carboxylate) functionality. For the fatty acid, the acid is present as a carboxylic acid. It is well established that an ester functionality is more easily reduced to a saturated hydrocarbon via hydrogenation technology than is a carboxylic acid functionality. This limits the amount of fatty acid that can be present in the feedstock and feedstock blends.
One method describes the conversion of depitched tall oil to a diesel fuel additive (see generally Canadian Patent 2,149,685). The method describes a hydrodeoxygenation process utilizing a hydrotreating catalyst. The catalyst is prepared by presulfiding. The sulfided nature of the catalyst can be maintained by adding sulfur to the tall oil feedstock at a level of 1000 ppm. The doping agent is carbon disulfide. The hydrodeoxygenation conversion is then performed at 410° C. and 1200 psi.
Another method describes the preparation of a diesel fuel from a vegetable TAG oil (see generally U.S. Patent Application 2007/0010682). The TAG oil is doped with 50 to 20,000 ppm sulfur. The hydrodeoxygenation step is performed between 580 and 725 psi and 305° and 360° C.